1. Field of the Invention
This invention is related in general to methods and devices for testing the efficacy of automotive brake fluid. In particular, the invention pertains to a novel approach based on a correlation between brake-fluid condition and the concentration of specific metals in the fluid, in particular copper, iron, and zinc.
2. Description of the Related Art
Brake fluid is used to transmit the pressure exerted on a motor vehicle""s brake pedal to the slave cylinders of the braking system. The most commonly used brake fluids consist of glycol-based liquids categorized as DOT3 and DOT4 on the basis of the boiling point resulting from their particular composition. In order to prevent boiling of the fluid caused by overheating during use, DOT3 and DOT4 fluids are required to have a dry boiling point (with no moisture in fluid) of at least 401xc2x0 F. (205xc2x0 C) and 446xc2x0F. (230xc2x0 C.), respectively, so that proper brake operation is ensured under all temperature conditions. A low boiling point can cause the brake fluid to vaporize under hot operating conditions, such as during continuous braking on a downhill road. Such vaporization can create pockets of compressible vapor in the system that dangerously reduce the effectiveness of the braking system. This is the occurrence normally referred to as xe2x80x9cbrake fadexe2x80x9d which, in extreme cases, can cause complete brake failure.
All types of glycol-based brake fluid are hygroscopic. As a result of this property, they readily absorb moisture that reduces their boiling point and, if unchecked, can become dangerous. Thus, replacement of DOT3 and DOT4 fluids is desirable when they have absorbed enough moisture to decrease their vaporization temperatures to about 284xc2x0 F. (140xc2x0 C.) and 311xc2x0 F. (155xc2x0 C.), respectively, which are minimum acceptable wet boiling points (with moisture absorbed by the fluid) empirically considered safe for brake operation. Since the moisture content of brake fluid increases with age and exposure to ambient humidity, it is clear that it should be checked periodically and that the fluid should be replaced when its boiling point approaches these safety limits.
The prior art shows a variety of inventions directed to the measurement of the boiling point of brake fluid in a vehicle in order to monitor its condition. For example, U.S. Pat. No. 5,380,091 and No. 5,785,425 to Buchanan describe boiling-point sensing devices for testing fluids, in particular hydraulic fluids such as automotive brake fluid. The devices include a portable probe containing a heated chamber to receive the fluid and a thermometer adapted to measure the temperature of the vapor generated by heating the fluid. The probe is immersed either in a container filled with a sample of brake fluid or in a vehicle""s brake-system reservoir to a depth sufficient to force the fluid into the chamber, where it is heated to vaporization to measure its boiling temperature.
The type of probe described in these patents has the disadvantage of requiring sampling of the fluid or access to a sufficiently deep reservoir to allow its immersion in the fluid, which is not always readily feasible. In addition, this kind of probe requires electrical power to heat the fluid to its boiling point, and at least two tests are recommended because of poor test repeatability at the relatively high temperatures of operation. Because the probes have a relatively long cycle time, in the order of 30-90 seconds, and require a cool-down time of several minutes between runs, each test is time consuming and therefore also expensive to perform. Therefore, these tests are typically not carried out for regular maintenance purposes and brake fluid is often used beyond its safe useful life.
Another approach to estimating the boiling point of brake fluid is by measuring its moisture content. Accordingly, a test strip has been developed that upon immersion in brake fluid turns to different color shades representative of the amount of water present in the fluid. One such test strip, sold by Wagner Brake Products under the trademark WETCHECK(copyright), is currently available for this purpose, but tests have shown that it is effective only at relatively high moisture concentrations (15-20%). In addition, the different shades of color, which vary from green to light brown, are not always easily distinguishable. Therefore, this test does not provide a readily available means for monitoring the boiling point of brake fluid.
These test strips used in the art to measure moisture also suffer from the disadvantage of being unstable and subject to rapid deterioration when unprotected from ambient moisture. Unless kept in a sealed container, the strips react with moisture in the air within a matter of hours or even minutes, depending on the humidity level, and therefore soon become wasted. Moreover, after use in a test, the color of the strip continues to gradually change until the darkest shade is reached, leaving only a limited time window for accurate measurement. This shortcoming also prevents a user, such a service mechanic, from being able to show the results of a test to a customer, thereby creating an issue of credibility regarding a purported need for replacement of the fluid.
Another important aspect of brake fluids is the corrosive nature of some of their constituents, which progressively damages metallic tubing and other parts of the brake system. In conventional fluids, amines are added to inhibit corrosion and prevent damage to metal parts that operate in contact with the fluid. As the brake fluid ages, its anticorrosive properties are measured in terms of reserve alkalinity, that is, the amount of amines remaining in the fluid to buffer the acidity resulting from breakdown of fluid constituents. Over time, thermal oxidation and volatization produce a significant reduction of the amine content and the concurrent decrease of anticorrosive properties. Tests have shown that the reserve alkalinity of DOT3 and DOT4 fluids is reduced to about 20 percent of its original value after 18 to 20 months of normal operation. Therefore, brake fluids also need to be checked and periodically replaced in order to prevent dangerous corrosion in the brake system. This invention is directed at a process and a device for establishing with a simple, inexpensive test the current condition of both the boiling point and the anticorrosive properties of a brake fluid in use.
One primary objective of this invention is a method and apparatus for determining whether the boiling point of a brake fluid in use is within allowable safety standards.
Another objective of the invention is a method and apparatus for assessing the condition of the fluid""s anticorrosive properties with the same test performed to determine its boiling point.
Another goal is a test that does not require sampling of the fluid and does not involve elaborate testing procedures.
Yet another goal is a procedure that can be carried out rapidly during regular automotive maintenance.
A final objective is a procedure that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objectives, the preferred embodiment of the present invention consists of a reactive test strip adapted to measure and indicate the concentration of copper ions in brake fluid in terms of a readily visible color change. The strip is immersed in the fluid and the resulting color acquired by reacting with the fluid is compared to a copper concentration-versus-color chart or to a standard color representative of the maximum concentration determined empirically to correspond to a boiling point considered safe for normal operation. If the color change indicates a higher copper concentration, the brake fluid is considered inadequate for safe operation and replaced without further tests. A similar strip can also be used to determine iron concentration. An automated embodiment of the invention includes an optical instrument to compare the color obtained from the test to a predetermined standard and determine whether a fluid change is recommended.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.